Three scenarios examine the design issues and threshold for when to use an oscillator vs a crystal
Have you ever thought about the actual cost of using a crystal versus a MEMS oscillator? This question may not be at the forefront of your selection process when the price of crystals seem so cheap—at least on the surface. But although the cost of crystal components is generally lower, once the total cost of design is calculated, the picture looks much different.
At SiTime, we hear from many customers who call us when they have crystal design issues such as cold startup failures, oscillator circuit problems from mismatched crystals, or failure to pass EMI tests. These problems cause engineering cost overruns during development and can create costly quality issues. Plus delaying the product release date can cause costly lost opportunities. Here we share three situations where customers came to SiTime to decrease their overall cost of ownership when facing crystal design concerns.
But first let’s briefly cover the basics … what is the difference between a crystal (XTAL) and an oscillator (XO)? A crystal (sometimes called a resonator) is a moving/resonating passive device that connects to the external oscillating circuit in the chip that it is timing, like an SoC, microcontroller or processor—as shown on the left below.
An oscillator, shown on the right, is an integrated timing solution that contains a resonator and an oscillator IC in one active device. In the case of SiTime oscillators, the resonator is based on silicon MEMS (micro-electro-mechanical systems) technology instead of the traditional quartz crystal. This architecture enables robust “plug-and-play” timing products that are flexible and very easy to design into a system.
Total cost of ownership
Oscillators are easier to design with since they include functionality and features that solve common and often difficult timing design problems, as illustrated in the following cases. These examples are based on pricing from Digi-Key for XTALs and XOs with the same output frequency, frequency stability, and package size. Added to the price of the timing component is the cost of engineering work-hours (based on $50 per hour) that is required to remedy the problem.
Each case has a different breakpoint based on production volume and engineering time. Not surprising the cost of designing with a crystal is lower when quantities are high and design costs are amortized over large volumes. Conversely, the cost of using an oscillator is lower when quantities are lower. But there’s more to the story.
What’s not factored into the following calculations is the opportunity cost due to project design delays, which can be tremendous in some markets. In some cases there are additional costs for outside services and testing—and these also can be significant. Plus there are other penalties such as the costs for additional materials/components for board re-spins, the cost of load capacitors that are required with crystals, and the additional board space consumed by the capacitors—all of which further tilt the equation toward using an oscillator.
For simplicity sake, in the following examples we’ve included ONLY the cost of the timing component and the engineering time to correct the crystal problem.
1. Cost of crystal vs oscillator – cold startup failure
Unlike crystals, MEMS oscillators do not have startup problems. In this case, 15 hours of engineering work was required to correct the crystal startup problem. Here, with a relatively quick fix, the cost benefit of using a MEMS oscillator is realized when production volume is around 2,800 units or less.
2. Cost of crystal vs oscillator – mismatched crystal causes oscillator failure
Because oscillators are an integrated solution (combining the resonator and oscillator IC in one package), matching errors are eliminated. Designers don’t need to worry about crystal motional impedance, resonant mode, drive level, oscillator negative resistance, or other pairing considerations. In this instance 40 hours of engineering work is required to correct the matching issue, making the cost of using an oscillator payoff at around 8,000 units or less.
3. Cost of crystal vs oscillator – EMI compliance failure
The clock is often the largest contributor to EMI (electromagnetic interference) in a system and it can cause a prototype to fail compliance testing. SiTime MEMS oscillators offer multiple techniques for quickly and easily reducing EMI. One such technique is spread spectrum clocking. Another feature is FlexEdge™, a programmable feature for adjusting the rise/fall time (slew rate) of the clock signal to lower EMI.
Crystals, on the other hand, don’t have these features. If designers need to use shielding or add a spread spectrum clock generator IC with their crystal, this adds expense and board space. Plus renting an anechoic chamber for additional testing could incur another $3,000. To redesign the board and retest, it can take 50 hours of engineering work, making it more beneficial to use a MEMS oscillator at volumes around 11,000 units or less. And this doesn’t include the additional materials and test facility costs mentioned above.
Bottom line – savings across the board
In addition to direct costs, there are other factors that affect the cost of designing with crystals. For example, oscillators can drive multiple loads. That means one oscillator can replace multiple crystals, which can provide a signal for only one device.
Additionally, SiTime MEMS oscillators are based on a programmable architecture that makes them readily available in any frequency, stability, and voltage within a very wide range. This provides great flexibility for designers in optimizing their design. In fact SiTime oscillators can be programmed by key distributors or even by customers in their own lab using the Time Machine II.
Programmability can also reduce the cost of qualification efforts if specification changes are needed. This time-saving benefit is possible because a MEMS oscillator (before programming) can generate millions of part numbers and specification combinations—all with the same base part.
Perhaps one of the biggest indirect savings comes in the form of higher reliability and quality. SiTime MEMS oscillators have higher reliability with over 1 billion hours MTBF (mean time between failure) compared to typical quartz devices with about 25 million MTBF. And our devices deliver less than 2 DPPM quality level which is about 30 times better than quartz devices. Plus SiTime MEMS oscillators have much better survival rates against shock and vibration compared to quartz crystals.
The higher failure rates of quartz crystals can increase costs in many ways such as the added resource costs for root-cause analysis or extra service and replacement costs. Plus the damage that quality issues do to a company’s reputation can have a huge and long-lasting negative effect on a company’s bottom line.
Using an oscillator in place of a crystal can lower costs in many ways. When procurement is focused on lowering component costs, remember that looking at the big picture will ultimately save in the long run. To learn more about the benefits of oscillators beyond cost, read our white paper: The top 8 reasons to use an oscillator instead of a crystal resonator.
 Based on Digi-Key pricing as of October 16, 2019 for oscillator and crystal with similar specifications:
a. SIT1602BI-21-XXE-25.000000D-ND oscillator with 25 MHz frequency output, ±20-ppm frequency stability, 3.2 x 2.5 x 0.75 mm package, -40 to 85°C operating temperature.
b. ABM8W-25.0000MHZ-4-D1X-T3 crystal with 25 MHz frequency output, ±20-ppm frequency stability and ±10-ppm initial frequency tolerance, 3.2 x 2.5 x 0.75 mm package, -40 to 85°C operating temperature.
 Based on $50 per work hour.
 Difference in cost between using an oscillator compared to a crystal with added engineering time included.
Thanks to Bruce Potvin, Vice President of Global Sales at SiTime, for his contributions to this article.
Related White Paper: